[OE-core] Tune files and knobs to turn

[Koen Kooi]

Hi,

We discussed tune files a bit during last nights TSC meeting and Khem had expressed the need before, so I'd like to get this discussion started by using armv7a as an example.

For armv7a capable cores we have the following hardware features:

* armv7a instruction set
* thumb1 instruction set
* thumb2 instruction set
* VFP coprocessor
* optional NEON coprocessor

For the ABI we can choose the following:

* softtp without hw support (e.g. no VFP instructions emitted, slow)
* softfp with hw support (e.g. VFP and/or NEON instructions emitted, fast)
* hardfp, emits VFP and/or NEON instructions, slightly faster than softfp/hw, incompatible with everything else

And the extra knobs:

* pure thumb1, no arm instructions (limited use)
* thumb1/arm interworking
* pure thumb2,  no arm instructions
* thumb2 interworking (not sure if that's actually usefull, thumb2 has complete coverage)

In OE .dev we have the following vars:

TARGET_FPU: switches between hw float and sw float, no reflection in package arch
ARM_FP_ABI: switches between softfp and hardfp, will create 'armv7a' or 'armv7a-hardfp' as package arch
ARM_INSTRUCTION_SET: switches between arm and thumb1, no reflection in package arch
THUMB_INTERWORK: turns on interworking, no reflection in package arch

(side note, oe-core/distroless and meta-yocto/poky don't turn set TARGET_FPU for armv7a and will generate slow code, angstrom does turn it on)

Khem and I would like to start building armv7a (and armv6) in pure thumb2 mode but we want to have the variables to turn those knobs make sense and be consistent. RP has expressed his desire to sort this all out before merging multilib. I'm sure x86/mips/ppc/etc have a similar need, so let's get this discussion started.

regards,

Koen